Alkali metal recovery from alkali metal amides



United States Patent 3,258,329 ALKALI METAL RECOVERY FROM ALKALI METAL AMIDES Lynn H. Slaugh, Pleasant Hill, and John H. Raley, Walnut Creek, Califi, assigiors to Shell Oil Company, New

York, N.Y., a corporation of Delaware No Drawing. Filed June 12, 1964, Ser. No. 374,837

6 Claims. (Cl. 75-66) This invention relates to an improved method for the recovery of alkali metal. More particularly, it relates to an improved method for producing certain alkali metals from the corresponding alkali metal amides.

Considerable recent emphasis has been placed on the development of processes wherein alkali metal, particularly sodium, is employed as a chemical reducing agent for unsaturated organic compounds, particularly as an agent for the partial reduction of aromatic compounds. In such a partial reduction process, exemplified by U.S. 2,182,242, issued December 5, 1939, to Wooster, alkali metal is reacted with an aromatic compound at low temperature in liquid ammonia in the presence of a hydrolytic agent, e.g., water or alcohol. Such processes, known as Birch reductions, efiect the partial reduction of the aromatic system, but have the attendantdisadvantage of converting the alkali metal to a chemically combined form, e.g., the hydroxide or the alkoxide, from which the alkali metal is not readily recoverable. In a more recent development, it has been found that reduction of certain aromatic compounds may be effected with alkali metal in liquid ammonia in the absence of a hydrolytic agent. In such instances the alkali metal is obtained at the conclusion of reaction as the alkali metal amide. It would be of advantage to provide an efficient method by which the alkali metal could be recovered-from the amide, and particular advantage would be gained if the amide were convertible to alkali metal and ammonia, thereby regenerating both original reactants of the reduction process. v

It is known that sodium amide may be converted by hydrogenolysis at low temperatures, below 200 C. for example, to sodium hydride and ammonia. Such a process is described by Levine et 211., Chem. Revs., 54, 449 (1954). At higher temperatures, circa 335400 C., the principal products arise from thermal decomposition of the sodium amide to the elements, that is, decomposition to sodium metal, nitrogen and hydrogen. Although it is known that alkali metal hydrides are pyrolyzable to the metal and hydrogen, see, for example, Hurd, Chemistry of the Hydrides, New York, John Wiley & Sons, Inc., 1952, the temperatures required for alkali metal hydride pyrolysis at reasonable rate are those at which extensive thermal decomposition of the corresponding alkali metal amide takes place, and thus an efficient onestep conversion of alkali metal amide to alkali metal and ammonia has not been considered to be feasible.

It is the object of this invention to provide an improved method for the recovery of alkali metal from the corresponding alkali metal amide. A more particular object is to provide a one-step process whereby alkali metal amide is converted to alkali metal and ammonia. A specific object is to provide a process for the conversion of alkali metal amide in the presence of molecular hydrogen, which process results in the production of alkali metal and ammonia and minimizes the thermal decomposition of the alkali metal amide to the elements.

3,258,329 Patented June 28, 1966 It has now been found that these objects are accomplished by the process of reacting certain alkali metal amides with molecular hydrogen at elevated temperature and pressure, which temperature and pressure is selected so that the thermal decomposition of the amide, normally the predominant mode of reaction at the temperatures employed, is no longer an important factor in-product determination, and the amide is converted substantially completely to the alkali metal and ammonia.

The term alkali metal as employed herein indicates certain members of Group IA of the Periodic Table, such as is found in The Merck Index, Rahway, N.I., Merck and Co., Inc., 1952, 6th ed. Although lithium is included as a member of Group IA in such tabular representations, from considerations of the reactivity of the metal and stability of its compounds, particularly the amide and the hydride, lithium is not similar to other members of Group IA. In any event, the conversion of lithium amide to lithium metal and ammonia does not observe the same principles as do the other alkali metal amides, and with regard to the present invention, lithium is not properly includable with the other members of Group IA. The term alkali metal as herein employed therefore refers to members of Group IA of the Periodic Table having an atomic number from 11 to 55, that is, sodium, potassium, rubidium and cesium, or mixtures thereof, and the amides of these metals are suitably employed as reactants in the process of the invention. By the term amide as employed herein is meant inorganic amide, that is, amide of the formula MNH wherein M is alkali metal having an atomic number from 11 to 55.

Although the process of the invention is operable for the conversion of rubidium amide and cesium amide, such materials are not frequently encountered, and from practical considerations, the utilization of amides of alkali metals having an atomic number from 11 to 19, that is, sodium amide and potassium amide, is preferred. The process is particularly suitable for the conversion of sodium amide to sodium and ammonia.

The process of the invention comprises contacting the alkali metal amide with molecular hydrogen at elevated temperature and pressure. Without wishing to be bound by any particular theory, it would appear that the products obtained are determined by the relative contribution of several overall competing reactions (no doubt the mechanisms are more complex) which are thought to occur under the process conditions employed. Employing the conversion of sodium amide for illustrative purposes, these possible competing reactions are represented by the equations below.

Equation 1 represents the thermal decomposition of sodium amide, which decomposition is known to occur at the reaction temperature employed. This reaction, although resulting in the production of sodium, is considered detrimental to the desired process, as the decomposition in this manner results in loss of potential ammonia through formation of elemental nitrogen and hydrogen. This reaction is irreversible. Equation 4 represents the desired overall reaction in which sodium amide is converted by the action of molecular hydrogen to the metal and ammonia. This reaction is not thought to be a direct reaction, but apparently results from proper control of the equilibria represented by Equations 2 and 3, and it should be noted that Equation 4 is the summation of the forward reaction (2) and the forward reaction (3).

Reaction (1), the forward reaction (2) and the forward reaction (3) are promoted by increased reaction temperature, so that increased thermal decomposition of the amide takes place when the elevated temperatures required to effect forward reaction (3) at a suitable rate are employed. The forward reaction (2) is promoted by increasing the hydrogen pressure, and although the effect of thermal decomposition can be reduced by the use of high hydrogen pressure, the forward reaction (3) is retarded by high hydrogen pressure so that utilization of excessive hydrogen pressures is detrimental from consideration of the overall reaction process, i. e., reaction (4). The choice of suitable reaction conditions therefore involves considerations of employing a reaction temperature suflicie-ntly high to facilitate hydrogenation of the amide without unduly promoting thermal .decomposition, and employing hydrogen pressures sufficient for effective amide hydrogenolysis, but not so high as to overly retard hydride decomposition.

The hydrogen pressures that are suitably employed may conveniently be related to the dissociation pressure at the reaction temperature employed of the hydride corresponding to the alkali metal amide undergoing conversion. Methods for calculating the dissociation pressure of the alkali metal hydrides are disclosed by Hurd, supra, chapter 4, and the references cited therein, wherein equations for calculating these dissociation pressures are provided. In the process of the invention, suitable hydrogen pressures do not exceed about 150% of the dissociation pressure of the intermediate alkali metal hydride at the reaction temperature employed, but are preferably at least of that pressure. Best results are obtained when the hydrogen pressure is from about 30% to about 110% of the dissociation pressure of the alkali metal hydride at the reaction temperature, and hydrogen pressures from about 50% to about 90% of the dissociation pressure are particularly suitable. It is within the contemplated scope of the invention, and is frequently very useful, to employ mixtures of hydrogen and-inert gas, e.g., methane, nitro- .ceeding the dissociation pressure, as the relative contribution of thermal decomposition of the amide is reduced when the hydrogen pressure is relatively high. For example, in the conversion of sodium amide at 335 C., the dissociation hydrogen pressure of the hydride is about 60 mm. and thermal decomposition at that pressure accounts for about 10% of the observed products. In contrast, in a similar conversion at 415 C., the dissociation hydrogen pressure of the hydride is about 550 mm., and thermal decomposition at that pressure accounts for only about 12% of the observed products. To allow utilization of suitably high hydrogen pressures, a reaction temperature \of at least about 375 C. is satisfactory, although a reaction temperature of at least about 400 C. is preferred. T he upper temperature limitation for effective utilization of the process of the invention is principally determined by practical and economic considerations. As the reaction temperature is increased, the allowable hydrogen pressure is increased also, and by use of higher hydrogen pressures it is possible to compensate for the concomitant increase in the rate of thermal decomposition of the amide. Little advantage appears to be gained, however, by utilization of temperatures over about 800 C.; the temperature range from about 425 C. to about 500 C. is particularly convenient.

The process is typically conducted by charging the alkali metal amide to al autoclave or similar reactor, and raising the temperature to the desired point prior to or simultaneously with the introduction of hydrogen. The alkali metal amide is contacted with hydrogen in any convenient manner, e.g., as by bubbling the hydrogen through the amide which is molten at reaction temperature. The gaseous efiluent, which is usually removed during the course of reaction may be cooled to condense the ammonia and any unrcacted hydrogen may be recycled. As the alkali metal is formed, it separates from the molten amide and may be Withdrawn periodically in a continuous process or removed subsequent to reaction if the process is conducted in a batchwise manner.

To further illustrate the process of the invention, the following examples are provided. It should be understood that they are not to be regarded as limitations, as the teachings thereof may be varied as will be understood by one skilled in this art.

Example 1 To an 80 ml. stainless steel autoclave was charged 10 g. of potassium amide. The vessel was heated to 435 C. and hydrogen, under 1 atmosphere of pressure was bubbled through the molten potassium amide from a gas inlet tube which extended to the bottom of the reactor. The ammonia was removed through a port in the top of the vessel and condensed by liquid nitrogen cooling.

As a measure of the extent of conversion of the amide to metal and ammonia, the condensed ammonia was titrated quantitatively with standard acid. In one experiment, 49% of the potassium amide was converted to 0 tassium metal during a two hour reaction period. In another similar experiment, conducted for seven hours, the conversion of the amide to potassium metal and ammonia was 84%.

EXAMPLE II In aseries of experiments following a procedure similar to that of Example I, 10 g. (256 milligram atoms) of sodium amide was hydrogenated at varying reaction temperatures and hydrogen pressures. The results of these experiments are shown in Table 1.

Table 1 Hydrogen pressure, Milligram atoms of Temperature, 0. mm. sodium amide hydrogenated per hour Similar results are obtained when rubidium or cesium amides are hydrogenated.

We claim as our invention:

1. The process of producing alkali metal by contacting the amide of an alkali metal having an atomic number from 11 to 55 with molecular hydrogen at a temperature above about 375 C. and at a hydrogen pressure of from about 10% to about of the dissociation pressure of the corresponding alkali metal hydride at said temperature.

2. The process of producing alkali metal by contacting the amide of an alkali metal having an atomic number from 11 to 55 with molecular hydrogen at a temperature from about 375 C. to about 800 C. and at a hydrogen pressure of from about 30% to about 110% of the dissociation pressure of the corresponding alkali metal hydride at said temperature.

3. The process of claim 2 wherein the alkali metal is sodium.

4. The process of claim 2 wherein the alkali metal is potassium.

5. The process of producing alkali metal by contacting the amide of an alkali metal having an atomic number from 11 to 19 with molecular hydrogen at a temperature 5 from about 400 C. to about 800 C. and a hydrogen pressure of from about 30% to about 110% of the dissociation pressure of the corresponding alkali metal hydride at said temperature.

6. The process of producing sodium by contacting sodium amide with molecular hydrogen at a temperature from about 425 C. to about 500 C. and a hydrogen pressure from about 30% to about 110% of the dissociation pressure of sodium hydride at said temperature, whereby the sodium amide and hydrogen react to form elemental sodium and ammonia.

References Cited by the Examiner DAVID L. RECK, Primary Examiner.

H. W. TARRING, Assistant Examiner. 

1. THE PROCESS OF PRODUCING ALKALI METAL BY CONTACTING THE AMIDE OF AN ALKALI METAL HAVING AN ATOMIC NUMBER FROM 11 TO 55 WITH MOLECULAR HYDROGEN AT A TEMPERATURE ABOVE ABOUT 375*C. AND AT A HYDROGEN PRESSURE OF FROM ABOUT 10% TO ABOUT 150% OF THE DISSOCIATION PRESSURE OF THE CORRESPONDING ALKALI METAL HYDRIDE AT SAID TEMPERATURE. 